Coupling of lasers to optical fibres

ABSTRACT

A laser is coupled to an optical fibre by coupling the laser beam to a planar waveguide, and coupling the waveguide to the fibre. Particularly the laser beam is coupled to the planar waveguide by a grating. For ease in obtaining and maintaining good alignment between waveguide and fibre core, the fibre is positioned in a groove in a support member. Crystallographic etching a Vee groove gives a simple and effective way of forming the groove in the support member.

"-T 1 United States 1 X3: 13.0 08 1 11 3,774,987

Boivin Nov. 27, 1973 1 COUPLING 0F LASERS T0 OPTICAL 3,610,727 10/1971UlI'iCh 350/96 we FIBRES 3,704,996 12/1972 Bomer et a1. 350/96 W6 [75]Inventor: Louis Philippe Boivin, Ottawa. FOREIGN PATENTS 0R APPLICATIONSOntario, Canada 1,807,574 7/1969 Germany 350/96 W6 [73] Assignee: BellCanada-Northern Electric primary E i -John K, C bi Research imited,Ottawa, Attorney-Sidney T. Jelly Ontario. Canada 221 Filed: June 27,1972 [57] ABSTRfCT A laser 1s coupled to an optical fibre by couplingthe [21] APPI- N04 2661674 laser beam to a planar waveguide, andcoupling the I waveguide to the fibre. Particularly the laser beam is 52US. Cl 350/96 wc, 350/162 R coupled to the planar waveguide y 11 grainsFor 9989 51 1m. (:1. G02b 5/14 in Obtainfng and maintaining goodalignment between 58 Field of Search 350/96 wo Waveguide and fibre core,the fibre is Positioned in a groove in a support member.Crystallographic etching 5 References Cited a Vee groove gives a simpleand effective way of form- UNITED STATES PATENTS ing the groove in thesupport member. 3,674,337 7/1972 Marcatili 350/96 W0 3 Claims, 6 DrawingFigures 3,674,335 7/1972 Ashkin et al.... 350/96 WG 3,614,198 10/1971Martin et al. 350/96 WG 3,674,336 7/1972 Kogelnik 350/96 WO A3 18 1 a I[I COUPLING OF LASERS 'I'O OPTICAL FIBRES This invention relates to thecoupling of lasers to optical fibres, particularly though notexclusively for optical communciation systems.

Lasers and optical fibres have both been used for communication systems,and the combinations of laser and optical fibre has been proposed.However, the efficient coupling of a laser to an optical fibre is aproblem which has not been adequately solved for practical systerns,particularly for single mode fibres.

A commonly used technique is to focus the laser beam onto the entranceface of the fibre by a microscope objective. If the laser beam isgaussian an efficiency of approximately 90 percent can be achieved.However with the GaAs lasers the small size of the cavity causes strongdiffraction efi'ects in the beam. The diffraction effects coupled with acomplex mode structure of the beam makes it difficult to focus the beamto a small spot for example of the order of 4 microns in diameter. Also,while focussing the beam by a lens may be acceptable for a laboratoryarrangement, it would not be acceptable in a practical system because ofthe high alignment tolerances required and the very shallow depth offocus which a very small focussed spot will involve. Both these factorsmake a system very susceptible to vibration, thermal expansion and otherdifficulties.

The present invention provides for the coupling of a laser to an opticalfibre in a relatively simple and efficient manner. The laser beam iscoupled to a planar waveguide and the waveguide focusses the beam to anoutput window closely coupled to an optical fibre.

Broadly the present invention comprises a .planar waveguide; an opticalfibre; means for coupling a laser beam to the planar waveguide; andmeans for supporting the optical fibre with its axis parallel to thewaveguide axis; the waveguide adapted to focus a beam from the laseronto the end of the optical fibre. A particularly convenient means forcoupling the laser beam to the planar waveguide is a grating.

The invention will be readily understood by the following description ofcertain embodiments, by way of example, in conjunction with theaccompanying diagrammatic drawings, in which:

FIG. 1 is a longitudinal cross-section through a complete system;

FIG. 2 is a plan view of the arrangement of FIG. 1;

FIG. 3 is a view similar to that of FIG. 2 but of an alternativearrangement;

FIG. 4 is a crosssection on the line IV-IV of FIG.

FIG. 5 is a plan view of a further alternative arrangement; and

FIG. 6 is a cross-section on the line VI-VI of FIG. 1.

As illustrated in FIG. 1, a laser beam 10 is directed onto a coupler 11in the present example a grating where the beam is coupled into a planarwaveguide 12. The beam passes along the waveguide as indicated at 13. Anoptical fibre l4, hereinafter referred to as a fibre, is mounted axiallyin line with the longitudinal axis of the waveguide. Fibre 14 has a core.15 which is the light conveying component. The light is retained withinthe core by internal surface reflection in the known manner. Thewaveguide 12 and fibre 14 are mounted on a substrate 16. The laser beam10 is of relatively large diameter. By being coupled into the waveguide12 it becomes a wide thin beam 13 and it is then necessary to focus thisbeam down to the approximate dimensions of the fibre core 15. FIG. 2illustrates one way of focussing the beam 13. In FIG. 2 the waveguide 12is of wedge-shape when viewed in the plane of the crosssection ofFIG. 1. After the laser beam 10 is coupled into the waveguide 12 by thegrating 11, the beam is focussed by internal reflection within thewaveguide. The angle of the wedge is quite small, for example the anglea is approximately 2 although slightly larger angles can be used, andalso smaller angles.

An alternative form of focussing is illustrated in FIGS. 3 and 4. A thinfilm lens 17 is formed on the waveguide 12. The lens 17 is formed byincreasing the thickness of the waveguide locally, as seen in FIG. 4,the increase in thickness having a lens form when viewed normal to theplane of the waveguide, as seen in FIG. 3. The lens 17 can be formed byfirst making the waveguide of a total thickness equal to the overallthickness of the lens and then removing material as by etchingto leavethe lens structure. An alternative method is to first form the waveguide12 to its correct thickness and then form the extra thickness of thelens by applying a further localized layer of the same material as thewaveguide. A further alternative is to form the extra thickness of thelens by applying an additional localized layer of material having anindex of refraction slightly lower than that of the main waveguidelayer. For example, with a main waveguide layer of index 1.6, theadditional layer could have an index of 1.55. Normal photo maskingtechniques can be used to make these thin film lenses. The additionalthickness at the lens 17 changes the effective index of the waveguide inthe lenticular region, causing focussing of the beam.

FIG. 5 illustrates a combination of the alternate forms of focussing asillustrated in FIGS. 2 and 3. The planar waveguide 12 is of twoportions, a tapered or wedge-shaped portion 124, as in FIG. 2, followedby a parallel portion 12b, as in FIG. 3. The grating 11 is coupled tothe wide end of the portion 120. A thin film lens 17 is formed on theparallel portion 12b, being of the same form as the lens 17 in FIG. 3.The parallel portion 12b is coupled to the fibre 14, as shown.

Having been focussed the beam is injected into the core 15 of the fibre14 by close coupling the end of the film to the end of the waveguide,the core 15 axially aligned with the waveguide. The coupling ofwaveguide to film can be improved under some conditions by chamferringthe end of the waveguide as shown at 18.

Considering now the individual items, the laser beam to waveguidecoupler 11 can be of various forms. For ease of fabrication and ease ofoperation a grating coupler is generally most convenient and can be madeas efficient as a prism coupler, which is an alternative. Basicallythere are two types of grating couplers --thin phase gratings and thickBragg-angle gratings. Thick Bragg-angle gratings can be made so as togive efficiencies of the order of 75 percent. Sinusoidal thin phasegratings are easier to make but have an efficiency normally of the orderof 35 percent. However, by using a relatively large angle of incidence,and adjusting the groove shape, blazing effects can be produced, to makesuch gratings approximately -80 percent efficient. For example, thegrating can have a saw-tooth crosssection and of a profile to suit theinstallation. One feature to be considered, for example, is the angle ofincidence of the laser beam onto the grating. The pitch of the gratingwill depend on various details to couple into the lowest order mode of athin film waveguide the pitch of the grating needs to be about 1 micron,at a wavelength of 0.63 micron, for example. The grating can be made byany convenient method, a typical method being by exposing photo-resistto interfering laser beams.

It is proposed that the substrate 16 be of silicon, for reasons to beexplained later. However other materials can be used, for example glassand various synthetic crystals such as GaAs and GaP. The waveguide 12,to be effective, must be on a substrate, or layer, having a refractoryindex lower than that of the waveguide. 1f the waveguide is deposited orotherwise placed on a substrate having a suitable refractory index valuethen the waveguide can be directly on the substrate. However if thesubstrate is not of material having an acceptable refractory index, anintervening layer is required. Thus, as illustrated in FIG. 1, withsubstrate 16 of silicon, it is necessary to provide a low index layer19. Suitable materials for the low index layer are SiO and an epoxyresin such as is made by Dow Corning under the reference XR-63-503. SiO,has a value for n approximately equal to 1.45 and the epoxy resinapproximately 1.40. To reduce losses due to interaction of theevanescent wave with the silicon substrate a thickness of about 2microns for the low index layer is preferred.

The high index layer that is the layer 12 in which the light istransmitted or conyeyed can also be of differing materials. One materialis a photo-resist known as KPR, having an index n=l.6l. The attenuationof this material is approximately 7db/cm at 0.63 micron decreasing toapproximately ldb/cm at 1 micron. An a1- temative is a lead-silicaspin-on oxide, having a controllable index of refraction n of 1.44 to1.66 and a lower attenuation coefficient of from about 0.5db/cm at 0.63microns to 0.3db/cm at 1 micron. An advantage of using a photo-resist asa waveguide material is the facility with which it can be used to makewaveguides of arbitrary shape; no etching or sputtering is required.Also it is very easy to make tapered edges which can be important forcoupling the light out of the waveguide or for reducing reflectionlosses at thin film lens interfaces for example.

With the materials described the thickness of the light conveying layerto support only the TEo or We modes must be between 0.14 and 0.58 micron(for 50 0.63 micron) or between 0.2 and 0.83 microns (for A =0.9micron). In practice the layer would be about 0.5 micron for A 0.63micron and about 0.7 micron for A 0.9 micron. It is easy to providelayers of these thicknesses with either XPR or SiO,-Pb spin-oxide.

The alignment of the fibre core 15 with the waveguide 12 is veryimportr- .rt. While various materials can be used for the substrate 16,as stated previously, silicon has been proposed as having certainadvantages. A

most important advantage is that silicon can be crystal- 60 bepositioned accurately to 1 micron. The etching will provide a channel orgroove having sides sloping iii towards the bottom of the channel.

FIG. 6 illustrates a cross-section through a channel 5 20 in thesubstrate 16. As channel 20 is etched it becomes progressively narrowerand if etching is permitted to carry on the sides 21 will meet at thebottom of an acute angle. The increase in width of the channel duringetching is extremely slow compared to the speed 10 of etching downwards.Therefore it will be appreciated that, knowing the external diameter ofthe fibre 14 it is very easy to decide on a desired width of channel togive a predetemiined position of the'fibre core 15. Crystallographicetching is self-limiting in that because 5 of the preferential directionof etching, time is not a critical factor. As long as etching iscontinued, to ensure that the bottom surface 22 will be clear of a fibrel4 resting in the channel 20, any further etching will have a verylimited effect on the position of the fibre as the width of the channelwill increase only by an extremely small amount, well within acceptabletolerances for positioning the fibre.

However, it is not necessary to restrict the substrate to silicon, othermaterials can be used and other con- 25 ventional methods used to formthe channel 20, which cated by the dotted line 23 in FIG. 1. To pennitclose coupling of the end face of the fibre 14 with the end of thewaveguide 12 this inclined surface would be etched away by analternative form of etching, of conventional form. ideally a sharp comeris preferred, but some undercutting, as indicated at 24 in FIG. 1, islikely to oc cur, but this will not affect the positioning of the fibre.However the inclined surface does not usually occur, its presencedepending upon the characteristics of the wafer.

The coupling efficiency between waveguide and fibre would be improved byimbedding the coupling region in a quasi index matching resin, n 1.5, toreduce reflection losses. What is claimed is:

1. Apparatus for coupling a laser to an optical fibre, comprising:

a planar thin film waveguide mounted on a silicon substrate; an opticalgrating coupler for coupling the laser beam into the waveguide;

focussing means for focussing the coupled beam at an exit of thewaveguide; and means for mounting an optical fibre with the core of thefibre substantially coaxial with the axis of the waveguide and alignedwith said exit, said means comprising a Vee-shaped channelcrystallographically etched in said silicon substrate. 2. Apparatus asclaimed in claim 1, wherein said focussing means comprises tapering thewaveguide toward the exit.

3. Apparatus as claimed in claim 1 wherein the exit of the waveguide ischamferred.

l l i l t

1. Apparatus for coupling a laser to an optical fibre, comprising: aplanar thin film waveguide mounted on a silicon substrate; an opticalgrating coupler for coupling the laser beam into the waveguide;focussing means for focussing the coupled beam at an exit of thewaveguide; and means for mounting an optical fibre with the core of thefibre substantially coaxial with the axis of the waveguide and alignedwith said exit, said means comprising a Vee-shaped channelcrystallographically etched in said silicon substrate.
 2. Apparatus asclaimed in claim 1, wherein said focussing means comprises tapering thewaveguide toward the exit.
 3. Apparatus as claimed in claim 1 whereinthe exit of the waveguide is chamferred.